Optical head device including an optically anisotropic diffraction grating and production method thereof

ABSTRACT

An object is to provide an optical head device whereby the light utilization efficiency can be increased and which can be produced at a low cost. A glass substrate 1 having projections and recesses 2 formed by patterning a SiON type transparent thin film on its inner surface by dry etching, and a second glass substrate 3 having a polyimide film 4 formed as an alignment film on its inner surface, are laminated with a space, the periphery is sealed by a sealing material made of an epoxy resin 5, and liquid crystal 6 is injected under vacuum into the interior to form an optically anisotropic diffraction grating. On the outer surface of this second glass substrate 3, a quarter-wave film 7 is laminated, and a third glass substrate 9 is bonded by a photopolymer 8 to obtain a diffracting element 10. This diffracting element 10 is disposed between a light source and an optical recording medium to obtain an optical head device.

TECHNICAL FIELD

The present invention relates to an optical head device for writingoptical information on or reading out optical information from opticaldiscs and magneto-optical discs such as CD (compact disks), CD-ROM andvideo discs, and a process for its production.

BACKGROUND ART

Heretofore, as an optical head device for writing optical information onor reading out optical information from optical discs and opticalmagnetic discs, one using a prism type beam splitter, and one using adiffraction grating or hologram element, as an optical part forintroducing (beam splitting) a signal light reflected from the recordingsurface of a disc to a detecting section, have been known.

Heretofore, the diffraction grating or the hologram element for anoptical head device has been prepared by forming a rectangular grating(relief) having a rectangular cross-section on a glass or plasticsubstrate by a dry etching method or an injection molding method,thereby to diffract light and provide a beam splitting function.

Further, in a case where it is attempted to improve the lightutilization efficiency over an isotropic diffraction grating whereby thelight utilization efficiency is about 10%, it is conceivable to utilizepolarization. For utilizing polarization, there has been a method ofincreasing the go and return efficiency by increasing the efficiency ofboth going (in the direction from the light source to the recordingsurface) and returning (in the direction from the recording surface tothe detecting section) by combining a quarter-wave sheet to a prism typebeam splitter.

However, the prism type polarized beam splitter is expensive, and othersystems have been sought. As one system, a method has been known inwhich a flat plate of berefringence crystal such as LiNbO₃ is used, andan anisotropic diffraction grating is formed on its surface to providedeflection selectivity. However, the berefringence crystal itself isexpensive, and it is difficult to apply such a method to the consumerproduct field.

With the isotropic diffraction grating, the utilization efficiency ingoing (in the direction from the light source to the recording surface)is about 50%, and the utilization efficiency in returning (in thedirection from the recording surface to the detecting section) is 20%,as mentioned above, whereby the go and return utilization is at a levelof 10% at best.

It is an object of the present invention to solve the above-describedproblems and to provide an optical head device which improves the lightutilization efficiency and which can be prepared at a low cost with highproductivity, and a process for its production.

DISCLOSURE OF THE INVENTION

The present invention provides an optical head device whereby writing ofinformation and/or reading out of information is carried out byirradiating light from a light source on an optical recording mediumthrough a diffracting element, wherein the diffracting element isprovided with an optically anisotropic diffraction grating havinglattice-like projections and recesses formed on the surface of atransparent substrate, and liquid crystal having an optical anisotropyfilled in said projections and recesses.

Further, the present invention provides a process for producing anoptical head device whereby writing of information and/or reading out ofinformation is carried out by irradiating light from a light source onan optical recording medium through a diffracting element, whichcomprises coating a transparent thin film on the surface of atransparent substrate for the diffracting element, then forminglattice-like projections and recesses on the transparent thin film by aphotolithographic method, and filling liquid crystal having an opticalanisotropy in said projections and recesses to form an opticallyanisotropic diffraction grating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transverse cross-sectional view of a basic structure of adiffracting element employing an optically anisotropic diffractiongrating by means of liquid crystal, representing Example 1.

FIG. 2 is a transverse cross-sectional view of a transparent substratefor an optical head device of Example 2.

FIG. 3 is a transverse cross-sectional view of a transparent substratefor an optical head device of Example 3.

FIG. 4 is a transverse cross-sectional view of a diffracting element ofan optical head device of Example 5.

FIG. 5 is a transverse cross-sectional view of a diffracting element ofan optical head device of Example 6.

FIG. 6 is a schematic view of a basic structure of the optical headdevice of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the above first invention, if the above projections and recesses aremade of an optically isotropic material and its refractive index issubstantially equal to the ordinary refractive index or extraordinaryrefractive index of the above liquid crystal, they function as anoptically anisotropic diffraction grating utilizing polarization oflight. It is particularly preferred that the refractive index issubstantially equal to the ordinary refractive index, since the rangefor selection of the transparent substrate material having suchprojections and recesses will thereby be wide, and a diffracting elementof good quality can be prepared at a low cost. Further, in a case wherethe refractive index of the projections and recesses is to be adjustedto be substantially equal to the extraordinary refractive index of theliquid crystal, one having a high refractive index (refractive index:1.58) such as a polycarbonate can be effectively used as the transparentsubstrate material.

Also in a case where the above projections and recesses are made of anoptically anisotropic material, and the refractive index n_(lg) of theprojections and recesses corresponding to the ordinary refractive indexof liquid crystal is substantially equal to the ordinary refractiveindex of the above liquid crystal, they function as an opticallyanisotropic diffraction grating utilizing polarization of light. Therefractive indices n_(lg) and n_(le) of the projections and recessescorresponding to the ordinary refractive index and the extraordinaryrefractive index of the above liquid crystal mean refractive indices inthe same directions as the directions showing the ordinary refractiveindex and the extraordinary refractive index of the liquid crystal (thedirections corresponding to the incidence direction and the polarizeddirection of light).

Here, if the refractive index n_(le) of the projections and recessescorresponding to the extraordinary refractive index of the liquidcrystal, is smaller than the above refractive index n_(lg) of theprojections and recesses, the difference between the extraordinaryrefractive index of the liquid crystal and the refractive index n_(le)of the projections and recesses, will be larger, whereby the depth ofthe diffraction grating required to obtain the desired diffractionefficiency, may be shallow, such being desirable.

Also in a case where the above projections and recesses are made of anoptically anisotropic material, and the refractive index n_(le) of theprojections and recesses corresponding to the extraordinary refractiveindex of the liquid crystal, is substantially equal to the extraordinaryrefractive index of the above liquid crystal, they function as anoptically anisotropic diffraction grating utilizing polarization oflight. Here, the refractive index n_(lg) of the projections and recessescorresponding to the ordinary refractive index of the liquid crystal, islarger than the above refractive index n_(le) of the projections andrecesses, the difference between the ordinary refractive index of theliquid crystal and the refractive index n_(lg) of the projections andrecesses, tend to be larger, whereby the depth of the diffractiongrating required to obtain a desired diffraction efficiency, may beshallow, such being desirable.

In the present invention, the structure may basically be such thatliquid crystal is filled in the recesses of the substrate having theprojections and recesses formed thereon. An optically anisotropicdiffraction grating is formed by the liquid crystal filled in therecesses and the projections of the substrate. It is preferred to use asubstrate having projections and recesses formed and a flat substratehaving no projections or recesses formed thereon, and to obtain adiffraction element having a structure of a liquid crystal cell havingliquid crystal filled therebetween.

It may also be that using a pair of transparent substrates havinglattice-like projections and recesses on their surfaces, the surfaceshaving the projections and recesses formed thereon, are arranged to faceeach other, and the above-mentioned liquid crystal is filled in theirprojections and recesses, and the pair of transparent substrates arelaminated to form an optically anisotropic diffraction grating. In sucha case, the respective projections and recesses may be shallow, andtheir preparation will be easy, such being preferred. Further, it ispreferred also in that the alignment of liquid crystal will be improvedby the two facing projections and recesses.

It is preferred that the above-mentioned pair of transparent substratesare laminated so that the projections and recesses formed on them wouldbe asymmetrical about the lamination plane, whereby a diffractiongrating having an asymmetrical cross-sectional shape can easily beprepared, and it is possible to make the diffraction efficiency ofeither one of ± first-order (diffraction) lights larger, so that thelight having the larger diffraction efficiency can be detected by asingle detector.

As the liquid crystal to be used in the present invention, a knownliquid crystal used for a liquid crystal display device such as nematicliquid crystal or smectic liquid crystal, can be used. Further, apolymer liquid crystal may also be used.

In the first invention, the difference An between the ordinaryrefractive index and the extraordinary refractive index of the aboveliquid crystal is preferably at least 0.05 and at most 0.35. If it isless than 0.05, the projections and recesses are required to be deep,whereby the production tends to be difficult, thus leading to a highcost. If it exceeds 0.35, the liquid crystal tends to be deteriorated bye.g. ultraviolet rays.

As the above transparent substrate, one having a refractive index offrom about 1.4 to about 1.6, such as glass, polyolefin or polycarbonate,is preferred, since it is readily adjustable to the ordinary refractiveindex of about 1.5 of the liquid crystal.

It is preferred that the above projections and recesses are made of anoptically isotropic material, and the temperature to satisfy DΔN=λ₀ /2is higher than 30° C., when n is the refractive index of the isotropicmaterial, D is the depth of the above projections and recesses, n_(g) isthe ordinary refractive index of the above liquid crystal, n_(e) is theextraordinary refractive index of the above liquid crystal, ΔN is thelarger one of |n-n_(g) | and |n-n_(e) |, and λ₀ is the wavelength invacuum of the light from a light source. Namely, when DΔN=λ₀ /2, the goand return efficiency by diffraction becomes highest, but ΔN variesdepending upon the temperature. Accordingly, in order to optimize thetemperature characteristic within a range of e.g. from 0 to 60° C., itis preferred to adjust the depth of the projections and recesses so asto satisfy the above relation at a temperature of at least 30° C.

Further, for the same reason as described above, it is preferred thatthe above projections and recesses are made of an optically anisotropicmaterial, and the temperature to satisfy DΔN=λ₀ /2 is higher than 30°C., when n_(lg) is the refractive index of the projections and recessescorresponding to the ordinary refractive index of the liquid crystal,n_(le) is the refractive index of the projections and recessescorresponding to the extraordinary refractive index of the liquidcrystal, D is the depth of the above projections and recesses, n_(g) isthe ordinary refractive index of the liquid crystal, n_(e) is theextraordinary refractive index of the liquid crystal, ΔN is the largerone of |n_(lg) -n_(g) | and |n_(le) -n_(e) |, and λ₀ is the wavelengthin vacuum of the light from a light source.

It is preferred to laminate a second transparent substrate made of glassor a plastic such as an acrylic resin, polyolefin or polycarbonate andhaving a thickness of about 1 mm on the optically anisotropicdiffraction grating of the above transparent substrate, so that a layerof liquid crystal can be sandwiched and fixed. The liquid crystal maynot only fill the projections and recesses, but a portion overflowedfrom the projections and recesses may form a thin layer of liquidcrystal between the transparent substrate and the second transparentsubstrate. If the distance between the projections on the surface of thetransparent substrate on which the projections and recesses are formed,and the second transparent substrate, is too large, the aligning forceof liquid crystal by an alignment film formed on the second transparentsubstrate tends to decrease. Accordingly, it is preferably at most 10μm, more preferably at most 5 μm. In a case where the second transparentsubstrate is laminated on the optically anisotropic diffraction gratingof the above transparent substrate, and a polyimide film (an alignmentfilm) for aligning liquid crystal is formed on the liquid crystal sideof the above second transparent substrate, the damage to the rubbingcloth during rubbing will be small, and the production cost will besmall, as compared with the case of forming a polyimide film on theabove transparent substrate.

It has been found that with the transparent substrate havinglattice-like projections and recesses, the projections and recessesthemselves have an aligning effect similar to the rubbed alignment film,and an adequate characteristic as a diffracting element can be obtainedby itself without providing an alignment film on the second transparentsubstrate. In such a case, substantially the same characteristic as inthe case where the alignment film is provided, can be obtained, and theproduction can be carried out at a low cost, since no alignment film hasto be provided.

Further, at the time of forming a polyimide film on the secondtransparent substrate, the rubbing direction for alignment and thedirection of stripes of the above projections and recesses (thelongitudinal direction of the lattice-like projections and recesses) arepreferably adjusted to be the same, whereby it is possible to improvethe stability and reproducibility of the alignment of the liquidcrystal, to increase ΔN and to prevent a deterioration of the alignmentratio due to the surrounding environment such as the temperature.

The cross-sectional shapes of the above lattice-like projections andrecesses are preferably bilaterally asymmetrical about a planeperpendicular to the longitudinal direction (stripe direction) of theprojections and recesses, whereby the diffraction efficiency of eitherone of + first-order (diffraction) light and - first-order (diffraction)light can be made larger than the other, and only the one having thelarger diffraction efficiency may be used so that a large go and returnefficiency can be obtained by means of a single detector. The bilateralasymmetrical shapes may, for example, be stepped shapes or sloping(saw-tooth) shapes.

Further, modifications may be made such that distances of theprojections and recesses have a distribution, some of the projectionsand recesses are made to be bilaterally asymmetrical and the rest aremade to be bilaterally symmetrical, or some of the projections andrecesses are made to be projections, and the rest are made to berecesses.

If a phase difference element such as a phase difference sheet or aphase difference film functioning as a half-wave sheet or a quarter-wavesheet, is laminated on the above optically anisotropic diffractiongrating of the transparent substrate, it is possible to let polarizationdirections cross as between the going direction of light (the directionfrom the light source side to the optical recording medium side) and thereturning direction of light (the direction from the optical recordingmedium side to the light source side) and thereby to let them functionas an optically anisotropic diffraction grating. As the above phasedifference sheet or phase difference film, a material such as apolycarbonate or polyvinyl alcohol having a thickness of from a few tensto a few hundreds micrometer, is preferably employed.

It is preferred that at least one side of the above phase differencesheet or phase difference film is covered by an organic resin such as aphotopolymer, or a thermosetting epoxy resin, or further, a thirdtransparent substrate such as a glass substrate or a plastic substratehaving a good flatness, is bonded by means of the above organic resin,whereby there will be merits such as a reduction of wave frontaberration and improvement of the reliability.

It is preferred that the periphery between the above transparentsubstrate and the second transparent substrate, or the periphery of theentire diffracting element, is sealed by a sealing material such as anepoxy resin, whereby not only leakage of liquid crystal can beprevented, but also undesirable physical or chemical changes of e.g.liquid crystal and organic resins due to changes in the humidity or thetemperature of the external environment, can be prevented.

A second diffraction grating which generates three beams for detecting atracking error, may be provided on the surface (the light source sidesurface) opposite to the surface of the above transparent substrate onwhich the projections and recesses are provided. In such a case,detection of a tracking error will be easy, such being desirable. Theabove-mentioned secondary diffraction grating may be formed by coating aphotopolymer or a photoresist, followed by exposure in a predeterminedpattern, or may be formed by directly processing the second substrate bya dry etching method.

Further, the projections and recesses on the inner side of one of thepair of transparent substrates, may be used as a diffracting element forgenerating such three beams.

FIG. 1 is a transverse cross-sectional view illustrating the basicstructure of the diffracting element of the present invention.

In FIG. 1, numeral 1 indicates a glass substrate which is a transparentsubstrate, numeral 2 indicates projections and recesses, numeral 3indicates a second glass substrate which is a transparent substrate,numeral 4 indicates a polyimide film which is an alignment film, numeral5 indicates an epoxy resin which is a sealing material, numeral 6indicates liquid crystal filled between a pair of substrates, numeral 7indicates a quarter-wave film as a phase difference film, numeral 8 is aphotopolymer, numeral 9 is a third glass substrate which is atransparent substrate, numeral 10 indicates a diffracting element,numeral 11 indicates a light incoming side (light source side), andnumeral 12 indicates a light outgoing side (optical recording mediumside).

FIG. 2 is a transverse cross-sectional view of a transparent substratefor an optical head device wherein the projections and recesses 2 arebilaterally asymmetrical saw-tooth shapes. FIG. 3 is a transversecross-sectional view of a transparent substrate for an optical headdevice wherein the projections and recesses 2 are bilaterallyasymmetrical stepped shapes.

FIGS. 4 and 5 are transverse cross-sectional views of a diffractingelement for an optical head device wherein the projections and recessesare formed by a transparent thin film.

In FIGS. 4 and 5, numeral 21 indicates a glass substrate which is atransparent substrate, numerals 22 and 32 indicate projections andrecesses, numeral 23 indicates a second glass substrate which is asecond transparent substrate, numeral 24 indicates a polyimide filmwhich is an alignment film, numeral 25 indicates a transparent adhesive,numeral 26 indicates liquid crystal filled between the pair ofsubstrates, numeral 27 is a quarter-wave film as a phase differencefilm, numeral 28 is a photopolymer, and numeral 29 is a third glasssubstrate which is a third transparent substrate.

The projections and recesses 22 in FIG. 4 show a state where thetransparent thin film remains only as projections, and at the recesses,the surface of the first glass substrate 21 as backing is exposed. Theprojections and recesses 32 in FIG. 5 show a state in which thetransparent thin film remains not only as projections but also at therecesses, and the surface of the first glass substrate 21 as backing isnot exposed.

FIG. 6 is a schematic view illustrating the basic structural embodimentof the optical head device of the present invention.

In FIG. 6, numeral 41 indicates a light source, numeral 42 indicates anoptically anisotropic diffraction grating, numeral 43 indicates a phasedifference element, numeral 44 indicates a diffracting elementcontaining the optically anisotropic diffraction grating 42 and thephase difference element 43, numeral 45 indicates an object lens,numeral 46 indicates an optical recording medium, and numeral 47indicates a detector. In this Figure, the projections and recesses ofthe diffraction element 44 are formed on the substrate on the opticalrecording medium side, and the longitudinal direction of the grating ofthe projections and recesses is directed perpendicular to the papersurface.

In the above description, lattice-like projections and recesses werereferred to for the description. However, as mentioned hereinafter, whensuch projections and recesses are to be formed by a transparent thinfilm, only projections may be formed by the transparent thin film, andin such a case, the projections and the recesses may have differentrefractive indices. In such a case, the refractive index of theprojections located at the same position as the liquid crystal will bequestioned. Accordingly, the refractive index of the projections andrecesses used in the above description may be interpreted as therefractive index of the projections.

When the optical head device of the present invention is used forreading out, a detector is usually provided on the light source side todetect reflected light from the optical recording medium. In order tocondense the reflected light in a desired beam shape on thelight-receiving surface of the detector, an in-plane curvature may beimparted to the optically anisotropic diffraction grating pattern of thediffracting element, or a distribution may be imparted to the gratinginterval. The above grating pattern may have a curvature distributionand a grating interval distribution designed by a computer and thus maybe made to be the optimum pattern for a focusing error detecting methodsuch as a spot size detection method. As the above detector, oneutilizing a semiconductor element such as a photodiode or a CCD element,is preferred as it is small in size and light in weight and of a lowpower consumption type.

By providing an anti-reflection film on the light incoming and outgoingsurfaces of the above diffracting element, loss of light can beprevented. In such a case, it is preferred to use an amorphous fluorineresin as the anti-reflection film, whereby film-formation can be carriedout at a low cost without using an expensive large size film-formingapparatus such as a vapor deposition apparatus.

In the present invention, it is preferred to provide a coating film of aphotopolymer such as a UV-curable acrylic resin on the light source sideof the diffraction element and/or the optical recording medium sidethereof, whereby wave front aberration caused by the irregularities onthe surface of the quarter-wave sheet or the glass substrate, can bereduced. Further, it is preferred to laminate a third transparentsubstrate such as a glass substrate or a plastic substrate having goodflatness on the coating film of photopolymer, whereby the wave frontaberration can remarkably be reduced. Thus, as the light incoming andoutgoing surfaces of the diffracting element are flattened, the wavefront aberration will consequently be reduced.

It is preferred to accommodate the above light source, the diffractingelement, the detector, the object lens, etc., in the same package,whereby it is possible to present an optical head device which is smallin size and easy for e.g. adjusting the optical axis. As the lightsource of the present invention, one utilizing a semiconductor elementsuch as a semiconductor laser (LD) or LED, is preferred. Particularlypreferred is LD, since it is small in size and light in weight and of alow power consumption type and has a coherence property. Further, as theoptical recording medium, an optical disc such as CD, CD-ROM or DVD(digital·video·disc), a magneto-optical disc, a phase change typeoptical disc, an optical card, or other recording medium for an opticalsystem for writing and/or reading out information by light, can be used.

The projections and recesses of the transparent substrate of the presentinvention may be formed by directly forming projections and recesses tothe substrate material or by providing a transparent thin film on thesurface of the substrate. In the case of directly forming projectionsand recesses to the substrate material, they may be formed by amechanical processing method such as cutting and grinding, orpress-molding, or by a chemical processing method such as etching. Inthe case of forming a thin film on the substrate material, they may beformed by a mechanical processing method such as cutting and grinding ofthe thin film or press-molding the thin film, by a chemical processingmethod such as etching of the thin film, or by a selective depositionmethod such as masking vapor deposition of the thin film material.

In the present invention, it is particularly preferred to form atransparent thin film on the surface of a transparent substrate. As sucha transparent thin film, various types of organic and inorganictransparent thin films may be used. In this manner, adjustment of therefractive index with liquid crystal can be made by the transparent thinfilm. The refractive index of this transparent thin film may usually beadjusted to the ordinary refractive index or extraordinary refractiveindex of liquid crystal to be used.

A transparent substrate having a high refractive index is hardlyavailable, and it is therefore difficult to adjust the refractive indexwith liquid crystal. Therefore, in order to adjust to the extraordinaryrefractive index of liquid crystal, only the transparent thin film maybe made to have a high refractive index, whereby the substrate may bemade of a material which is inexpensive, reliable and durable, such as ausual glass substrate. Specifically, when the transparent substrate is ausual glass substrate, the refractive index is at a level of 1.5, andone having a larger refractive index is used.

Specifically, it may be formed by using a transparent material selectedfrom SiO_(x) N_(y) (0≦x<2, 0≦y<1.3, refractive index: about 1.5 to 1.9),MgO (refractive index: 1.72), PbF₂ (refractive index: 1.75), Y₂ O₃(refractive index: 1.87) and a mixture of Al₂ O₃ and ZrO₂ (refractiveindex: 1.63 to 2.05), or a photosensitive organic material such as aphotosensitive polyimide (refractive index: 1.78). Further, a glass filmformed by a sol-gel method, or a glass film formed by firing a frit, mayalso be used.

Among such transparent thin films, it is preferred to use a materialcomposed of SiO_(x) N_(y) (0≦x<2, 0≦y<1.3). With this material, therefractive index can be adjusted within a wide range of from about 1.5to 1.9 by suitably adjusting the ratio of x to y, and it can readily beadjusted to the ordinary refractive index or extraordinary refractiveindex of liquid crystal. Further, fine processing can readily be carriedout by a dry etching method, such being desirable.

More preferably, the refractive index of the above transparent thin filmor the projections is at a level of from 1.7 to 1.9, so that it issubstantially equal to the extraordinary refractive index (about 1.8) ofliquid crystal, and it efficiently functions as a diffraction grating toS-waves as mentioned above. As a transparent substrate having arefractive index which is substantially equal to the ordinary refractiveindex (about 1.5) of liquid crystal, a glass substrate or a plasticsubstrate may, for example, be preferably employed.

It is preferred that the above transparent thin film is a SiO_(x) film(1<x<2), whereby it is readily possible to prepare projections andrecesses having a refractive index which substantially agrees to theordinary refractive index or extraordinary refractive index of liquidcrystal. Further, the above transparent thin film may be made of anorganic resin such as an acrylic resin, so that the projections andrecesses may be formed by means of a 2P method (a photopolymerizationmethod) or a selective photopolymerization method.

Specifically, the projections and recesses are formed as follows. Aphotoresist is coated by e.g. a spin coating method, on the surface of atransparent substrate such as a polished glass substrate. A photomaskhaving a predetermined pattern is intimately put on the photoresistfilm, followed by exposure with ultraviolet rays and further byphotoresist development treatment to form a lattice-like pattern of thephotoresist on the surface of the transparent substrate. Using thelattice-like pattern of the photoresist further as a mask, dry etchingis carried out by means of a gas such as CF₄ to form lattice-likeprojections and recesses for an optically anisotropic diffractiongrating, having a depth of from 1 to 6 μm and a pitch of from 2 to 50μm, particularly a depth of from 1 to 2 μm and a pitch of from 2 to 20μm.

Otherwise, using the transparent substrate prepared by the above methodas a master substrate, an acrylic resin or the like may be cast andmolded, or a mold may be prepared based on the above transparentsubstrate, and a transparent substrate having lattice-like projectionsand recesses may be prepared by an injection molding method or a 2Pmethod by using a material such as an acrylic resin, a polyolefin, apolycarbonate or a polyethersulfone.

Here, for example, a glass substrate may directly be subjected to dryetching. However, there is a problem that the etching speed is slow andit is difficult to form a constant depth with good reproducibility, orit is difficult to reduce the depth distribution.

Therefore, a film having a refractive index close to the refractiveindex of the transparent substrate such as a glass substrate, such as aSiO₂ film, may be formed by e.g. a vapor deposition method so that thedesired depth of projections and recesses can be obtained, whereby dryetching can be carried out with good reproducibility and with a smallface distribution by utilizing the difference in the etching speedbetween the transparent substrate and the SiO₂ film. In a case where atransparent thin film and a substrate having a small difference in therefractive index are to be laminated, it is preferred to adjust thedifference between the refractive index of the transparent substrate andthe refractive index of the transparent thin film such as the SiO₂ filmto be within 0.1 in order to prevent e.g. undesirable reflection by theinterface. This does not apply in a case where a transparent thin filmhaving a high refractive index is to be laminated on a substrate havinga low refractive index, to adjust the refractive index to theextraordinary refractive index of liquid crystal.

The refractive index of the above SiO₂ film is usually about 1.46, whichis lower than the ordinary refractive index of liquid crystal, wherebyit is not easy to obtain a good characteristic. Further, the ordinaryrefractive index of liquid crystal is usually lower than theextraordinary refractive index. Accordingly, by the following method,the projections and recesses having a refractive index whichsubstantially agrees to the ordinary refractive index of liquid crystal,can easily be prepared.

Namely, a SiO_(x) film (1<x<2) can be formed by film-forming a mixtureof SiO having a high refractive index and SiO₂ having a low refractiveindex, by e.g. a vapor deposition method. Otherwise, a SiO_(x) film(1<x<2) can be formed by gradually increasing the oxygen partialpressure in the atmosphere gas in the vapor deposition apparatus duringfilm-forming SiO by a vapor deposition method. This SiO_(x) film is anoxide of Si. Accordingly, when CF₄ is, for example, used as the etchinggas, the dry etching speed tends to be high due to the high volatilityof CF₄, and the etching selectivity ratio with the glass substrate tendsto be good. When the SiO_(x) film is employed, it is possible to bringthe refractive index of the projections and recesses or the projectionsto substantially the same level as the ordinary refractive index orextraordinary refractive index of liquid crystal.

The SiO_(x) N_(y) film can be made a film having a refractive indexclose to the ordinary refractive index or extraordinary refractive indexof liquid crystal by properly selecting x and y. Such a SiO_(x) film ora SiO_(x) N_(y) film is readily precisely processable by a dry etchingmethod, such being preferred.

The SiO_(x) N_(y) film has a merit in that not only the refractive indexcan be controlled by the proportions of x and y, deterioration of theoptical properties such as light absorption can be prevented within arelatively wide proportional range by adjusting y>0. Further, in thecase of controlling the oxidation number as in the case of SiO_(x),there is a problem in the reproducibility and stability in massproduction. Whereas, in the case of SiO_(x) N_(y) where y>0, the valenceelectrons are basically close to a satisfied condition, whereby there isa merit that the reproducibility, stability and reliabilities are allexcellent.

Specifically, to bring the refractive index of the transparent thin filmto a level of the ordinary refractive index (about 1.48 to 1.54) ofliquid crystal, x and y may be adjusted at a level of 1.65≦x≦1.85 and0<y≦0.2. Further, in order to bring the refractive index of thetransparent thin film to a level of the extraordinary refractive index(about 1.55 to 1.8) of liquid crystal, x and y may be adjusted to alevel of 0.6≦x≦1.65 and 0.2≦y≦0.9. The values of x and y variesdepending upon the forming method or the forming conditions of atransparent thin film, and they may be optimized by experiments.

As a method for forming SiO_(x) N_(y), a plasma CVD method is preferablyemployed. However, a reactive direct current sputtering method whereinsputtering is carried out in an atmosphere having O₂ gas, N₂ gas and N₂O gas mixed in the predetermined proportions using an electricallyconductive Si substrate as a target, is more preferred, since the filmforming rate is high as compared with the plasma CVD method.

Specifically, the projections and recesses are formed as follows. Atransparent thin film is formed on the surface of a transparentsubstrate such as a polished glass substrate by a plasma CVD method or areactive direct current sputtering method. Specifically, SiO_(x) N_(y)film is formed by adjusting the proportions of oxygen and nitrogen so asto bring the refractive index close to both the ordinary refractiveindex of liquid crystal and the refractive index of the transparentsubstrate (the refractive index of both being about 1.5), or close tothe extraordinary refractive index of liquid crystal.

Then, a photoresist is coated by e.g. a spin coating method on theSiO_(x) N_(y) film, and a photomask having a predetermined pattern isintimately put on the photoresist film, followed by exposure withultraviolet rays and then by photoresist development treatment to form alattice-like pattern of the photoresist on the surface of thetransparent substrate. Using the lattice-like pattern of the photoresistas a mask, dry etching is further carried out by means of a gas such asCF₄, C₂ F₆, C₃ F₈ or CHF₃ to form lattice-like projections and recessesfor an optically anisotropic diffraction grating, having a depth of from1 to 6 μm and a pitch of from 2 to 50 μm.

In the foregoing, a single diffracting element has been described.However, it is for example, possible that diffracting elements having aplurality of liquid crystals filled on a transparent substrate of a sizeof 120×120 mm, are formed, and they are finally individually separated.With respect to a case wherein the refractive index of the lattice-likeprojections and recesses is adjusted to substantially agree to theordinary refractive index of liquid crystal, and the liquid crystal isnot twisted, the operation will be explained.

Liquid crystal is aligned in a direction (a direction perpendicular tothe paper surface in FIG. 1) substantially parallel with thelongitudinal direction of the lattice-like projections and recesses.However, the liquid crystal and the projections and recesses have anequal refractive index to P-wave (the polarized light component having apolarization direction which is in parallel with the paper surface inFIG. 1) entered from the light source (from below in FIG. 1). Namely,the optically anisotropic diffraction grating becomes transparent toP-wave. Therefore, P-wave receives no change and enters into thequarter-wave sheet as it is and changes into a circularly polarizedlight, which passes through an aspherical lens as the object lens, andlight of substantially 100% will reach the recording surface of theoptical recording medium.

A reflected light reflected by the above recording surface and returnedby passing again through the aspherical lens, passes again through thequarter-wave sheet and changes into S-wave (a polarized light componenthaving a polarization direction which is perpendicular to the papersurface in FIG. 1) having a polarization direction which is different by90°. When S-wave enters into the optically anisotropic diffractiongrating, the refractive indices of the liquid crystal and theprojections and recesses are different this time, and they function as adiffraction grating, whereby a diffraction efficiency will be obtainedat the maximum of 40% as + primary light and at the maximum of about 40%as - primary light. A go and return efficiency of 40% is obtainable whena detector for detecting + primary light or - primary light is disposedat either side, and a go and return efficiency in a total of 80% can beobtained when detectors are provided at both sides.

Further, when the above projections and recesses are formed to havesloping shapes (saw-tooth shapes), a go and return efficiency of fromabout 70 to 90% is obtainable, and when they were formed to have steppedshapes of three steps, a go and return efficiency of about 81% isobtainable.

In such a case, in the going direction, no significant difference isobserved irrespective of on which substrate the grating is formed.However, in the diffracted return direction, a higher diffractionefficiency is observed by providing the grating on the substrate on theoptical recording medium side than by providing the grating on the lightsource side. The reason for this is not known. However, it is consideredthat when the grating is provided on the light source side, the lightpasses through liquid crystal (optically anisotropic material) and thenreaches the diffracting section, and the light may receive some opticalchange (disturbance) during this period.

When the refractive index of the lattice-like projections and recessesis made to substantially agree to the extraordinary refractive index ofliquid crystal, when S-wave (light having a polarization directionperpendicular to the paper surface in FIG. 1) from the light source ispermitted to enter, the light advances straight in the going directionand is diffracted in the returning direction in the same manner asdescribed above.

Here, when the refractive index of the first glass substrate is about1.5 which is substantially equal to the ordinary refractive index ofliquid crystal, and the transparent thin film is present only at theprojections, and the refractive index of the projections issubstantially equal to the extraordinary refractive index of liquidcrystal, the liquid crystal and the projections of the transparent thinfilm will be in contact with the first glass substrate. Due to thedifference of 0.3 in the refractive index between the transparent thinfilm and liquid crystal, and the first and second glass substrates,reflection occurs at the interfaces. The reflectance at the aboveinterface is about 0.8% as calculated per interface. The transmittancetaking such reflection loss into consideration is 98.4% with twosurfaces, and the loss will be 1.6%.

Further, also in the return direction, reflectance by the interface willoccur at the optically anisotropic diffraction grating. It is difficultto accurately estimate this reflection loss, but it is calculated on theassumption that the area of the projections is substantially one half,and one half of light is reflected at the two interfaces. In such acase, the transmittance of light will be 99.2%, and the reflection losswill be 0.8%. From the foregoing, in the total of the going andreturning directions, the transmittance will be 97.6%, and thereflection loss is assumed to be 2.4%. However, even in the case of theabove P-wave input, a reflection loss of 1.6% is unavoidable, and thedifference from the S-wave input in the present invention is believed tobe slight and at a practically no problematic level.

Further, there is a case where a thin film of the transparent materialremains at the bottom of the recesses of the projections and recessesfor a reason of practical production. In such a case, the reflectionloss is assumed to be about 3.2%. Even such a construction may beemployed.

In the above example, the liquid crystal is not twisted, but the liquidcrystal may be twisted so long as the lattice-like projections andrecesses correspond to the polarization direction.

EXAMPLES Example 1

The structure of Example 1 is shown in FIG. 1. On one surface of a glasssubstrate 1 having a relatively small alkali component having athickness of 1 mm and a size of 10×10 mm and a refractive index of 1.54,lattice-like projections and recesses 2 having a rectangularcross-section and having a depth of 1.55 μm and a pitch of 9 μm, wereformed by a photolithography method and a dry etching method.

Specifically, on one surface of a glass substrate 1 having both surfacespolished, a photoresist is coated by a spin coating method. Then, aphotomask having a predetermined pattern is intimately put on thephotoresist film, followed by exposure with ultraviolet rays and then byphotoresist developing treatment to form a lattice-like pattern of thephotoresist on the surface of the transparent substrate. Using thelattice-like pattern of the photoresist as a mask, dry etching wasfurther carried out by means of CF₄ gas to form the projections andrecesses.

A polyimide film 4 was formed as an alignment film for aligning liquidcrystal, on one surface of a second glass substrate 3 having arelatively small alkali component, and rubbing treatment for alignmentwas carried out. The two glass substrates were laminated so that thesurface of the above glass substrate 1 on which the projections andrecesses 2 were formed and the surface of the second glass substrate 3on which the polyimide film 4 was formed, faced each other, and therubbing direction of the polyimide film 4 and the stripe direction ofthe above projections and recesses 2 were directed in the samedirection, and the periphery of the two glass substrates was sealed withan epoxy resin 5 containing a spherical spacer having a diameter ofabout 4 μm, except for the inlet for injecting liquid crystal.

Liquid crystal 6 (nematic liquid crystal, P-008, tradename, manufacturedby Merck Co., ordinary refractive index: 1.525, extraordinary refractiveindex: 1.771) was vacuum-injected from the inlet for injecting liquidcrystal. At that time, ΔN=0.23, D=1.55 (μm), λ₀ =678 (nm), and D was setso that DΔN=λ₀ /2 was satisfied at 35° C.

On the surface on the opposite side of the polyimide film 4 of the abovesecond glass substrate 3, a quarter-wave film 7 was laminated and bondedby a transparent adhesive, and further a photopolymer 8 to reduce thewave front aberration and a third glass substrate 9 were laminated andbonded thereon to obtain a diffracting element 10. An anti-reflectionfilm by means of a dielectric multilayer film was applied at the lightincoming portion 11 from a light source and a light outgoing portion 12of the diffracting element 10.

As the light source, a semiconductor laser (to be provided below thediffracting element 10 in FIG. 1, not shown) was used, and when P-wave(a polarized light component parallel to the paper surface) having awavelength of 678 nm was permitted to enter, the transmittance of P-wavewas about 97%. Further, the reflected light (circularly polarized light)from an optical disc (to be provided above the diffracting element 10 inFIG. 1, not shown) was changed to S-wave (a polarized light componentperpendicular to the paper surface) by the quarter-wave film 7. ThisS-wave was diffracted by the optically anisotropic diffraction grating,whereby the diffraction efficiency of + first-order (diffraction) lightwas 33%, and the diffraction efficiency of - first-order (diffraction)light was 33%. As a result, the going efficiency was about 97%, and thego and return efficiency was about 64% (± first-order (diffraction)light detection).

Example 2

Preparation was carried out in the same manner as in Example 1 exceptthat the cross-sectional shapes of the projections and recesses formedin the glass substrate 1 were changed to saw-tooth shapes as shown inFIG. 2. In Example 2, the transmittance of P-wave was about 97%, thediffraction efficiency of + first-order (diffraction) light of S-wavewas about 75%, and the diffraction efficiency of - first-order(diffraction) light was about 2%. As a result, the going efficiency wasabout 97%, and the go and return efficiency was about 73% (± first-order(diffraction) light detection).

Example 3

Preparation was carried out in the same manner as in Example 1 exceptthat the cross-sectional shapes of the projections and recesses formedin the glass substrate 1 were changed to stepped shapes of three stepsas shown in FIG. 3. In Example 3, the transmittance of P-wave was about79%, and the diffraction efficiency of + first-order (diffraction) lightof S-wave was about 70%, and the diffraction efficiency of - first-order(diffraction) light was about 2%. As a result, the going efficiency wasabout 97%, and the go and return efficiency was about 68% (± first-order(diffraction) light detection).

Example 4

The structure was the same as in Example 1 except that no polyimide film4 was formed. In this case, liquid crystal 6 was aligned solely by thealigning force of the projections and recesses 2. In Example 4, thetransmittance of P-wave was about 97%, and the diffraction efficiencyof + first-order (diffraction) light of S-wave was about 31%, and thediffraction efficiency of - first-order (diffraction) light was about30%. As a result, the going efficiency was about 97%, and the go andreturn efficiency was about 59% (± first-order (diffraction) lightdetection).

Example 5

Example 5 is shown in FIG. 4. On a first glass substrate 1 having a sizeof 10 mm×10 mm, a thickness of 0.5 mm and a refractive index of 1.52, atransparent thin film of SiO_(x) (x≈1.2) having a refractive index of1.8 and a depth of 1.2 μm, was formed by a vacuum deposition method.Then, the transparent thin film of SiO_(x) (x≈1.2) was formed intoprojections having a pitch (cycle) of 10 μm by a photolithographicmethod and a dry etching method thereby to form lattice-like projectionsand recesses 22 having rectangular cross-sectional shapes in a planeperpendicular to the longitudinal direction. By this etching, thetransparent thin film remained only at the projections, and at therecesses, the glass substrate surface was exposed.

A second glass substrate 23 having a size of 10 mm×10 mm, a thickness of0.5 mm and a refractive index of 1.52, was prepared, and on its surfaceon the side in contact with liquid crystal 26, a polyimide alignmentfilm 24 was formed. The first and second glass substrates 21 and 25 werelaminated and bonded so that the rubbing direction was along the stripedirection of the above projections and recesses 22. At that time, theperiphery of the two glass substrates was sealed except for an openingfor injection of liquid crystal.

Specifically, the procedure was as follows. An epoxy resin (not shown inFIG. 4) containing a 4 μm spherical spacer was coated along theperiphery of the first glass substrate 21, and the second glasssubstrate 25 was mounted thereon. Thereafter, in a reduced pressureatmosphere, a mixed liquid crystal composition (BL009, tradename,manufactured by Merck Co., nematic liquid crystal, Δn=0.2915, ordinaryrefractive index=1.5266, extraordinary refractive index=1.8181, phasetransition temperature to solid liquid crystal phase≦-20° C., phasetransition temperature to isotropic phase=108° C.) was injected asliquid crystal 26. The above opening was closed with a sealing resin toobtain an optically anisotropic diffraction grating.

Then, on the second glass substrate 23 (on the surface on the oppositeside of the projections and recesses), a phase difference film 27 madeof a polycarbonate, was bonded by means of a transparent adhesive 25.Further, a UV curable acrylic resin 28 was coated thereon. Further, athird glass substrate 29 was mounted thereon, followed by irradiationwith ultraviolet rays to laminate and bond the third glass substrate 29.Further, with respect to the entire element, an anti-reflection film wasformed on the light-incoming surface and the light-outgoing surface toobtain a diffracting element.

As a result of the foregoing, the transmittance was 95% to S-wave (lightin the polarization direction perpendicular to the paper surface in FIG.4) having a wavelength of 678 nm from a semiconductor laser (not shownin FIG. 4). To P-wave (light in the polarization direction parallel tothe paper surface) from an optical disc (not shown in FIG. 4), thediffraction efficiency of first-order (diffraction) light was 36.9%, andthe diffraction efficiency of - first-order (diffraction) light was34.0%.

Thus, the go and return efficiency was 67.3%, which was a practicallysufficiently high efficiency. Further, the wave front aberration oftransmitted light was at most 0.015 λ_(rms) (root mean square) at thecenter portion (within a circle having a diameter of 2 mm) of the lightincoming and outgoing surfaces of the diffracting element.

Example 6

Example 6 is shown in FIG. 5. The same parts as in Example 5 areindicated by the same symbols. On a first glass substrate 1 having asize of 10 mm×10 mm, a thickness of 0.5 mm and a refractive index of1.52, a photosensitive polyimide (Photonece, tradename, manufactured byToray Co., refractive index: 1.78) was coated in a thickness of 1.3 μmby a spin coating method. Thereafter, the photosensitive polyimide filmwas formed into projections with a pitch (cycle) of 8 μm by aphotolithographic method thereby to form lattice-like projections andrecesses 32 having rectangular cross-sectional shapes in a planeperpendicular to the longitudinal direction. Thereafter, rubbingtreatment was carried out in the stripe direction of the aboveprojections and recesses 32. The subsequent steps were carried out inthe same manner as in Example 5 to obtain a diffracting element.

As a result of the foregoing, the transmittance was 90% to S-wave (lightin a polarization direction perpendicular to the paper surface in FIG.5) having a wavelength of 678 nm from a semiconductor laser (not shownin FIG. 5). To P-wave (light in a polarization direction perpendicularto the paper surface in FIG. 5), the diffraction efficiency offirst-order (diffraction) light was 34.6%, and the diffractionefficiency of - first-order (diffraction) light was 31.9%.

Thus, the go and return efficiency was 59.8%, which was a practicallysufficiently high efficiency. Further, the wave front aberration oftransmitted light was at most 0.015 λ_(rms) at the center portion(within a circle having a diameter of 2 mm) of the light incoming andoutgoing surfaces of the diffracting element.

Example 7

In the same manner as in Example 6, the structure of FIG. 5 wasemployed. On a first glass substrate 21 having a size of 10 mm×10 mm, athickness of 0.5 mm and a refractive index of 1.52, SiO_(x) N_(y)(x=0.7, y=0.8) was formed into a film having a thickness of 1.4 μm byreactive sputtering. The film-forming conditions were such that thesubstrate temperature was 200° C., a mixed gas was used with a nitrogengas flow rate of 17.4 SCCM and an oxygen gas flow rate of 0.2 SCCM, andthe gas pressure during the film formation was 5×10⁻³ torr.

The formed film was 680 nm and had a refractive index of about 1.8, andno absorption was observed to the wavelength of at least 600 nm.Thereafter, a transparent thin film of SiO_(x) N_(y) (x=0.7, y=0.8) wasformed into projections with a pitch (cycle) of 10 μm by aphotolithographic method and a dry etching method thereby to formlattice-like projections and recesses 32 having rectangularcross-sectional shapes in a plane perpendicular to the longitudinaldirection. Thereafter, a polyimide film was coated on both substrates,and rubbing treatment was carried out in the stripe direction of theprojections and recesses 32. The subsequent steps were carried out inthe same manner as in Example 5 to obtain a diffracting element.

As a result of the foregoing, the transmittance was 80% to S-wave (lightin the polarization direction perpendicular to the paper surface in FIG.5) having a wavelength of 678 nm from a semiconductor laser (not shownin FIG. 5). To P-wave (light in a polarization direction perpendicularto the paper surface in FIG. 5) from an optical disc (not shown in FIG.5), the diffraction efficiency of first-order (diffraction) light was33.7%, and the diffraction efficiency of - first-order (diffraction)light was 32.8%.

Thus, the go and return efficiency was 53.1%, which was a practicallysufficiently high efficiency. Further, the wave front aberration oftransmitted light was at most 0.015 λ_(rms) at the center portion(within a circle having a diameter of 2 mm) of the light incoming andoutgoing surfaces of the diffracting element.

Further, this SiO_(x) N_(y) (x=0.7, y=0.8) film can also be formed by aplasma CVD method using SiH₄, N₂ O and NH₃, by controlling the flow rateratio of N₂ O to NH₃ to a level of 1:24.

Example 8

In the same manner as in Example 7, SiO_(x) N_(y) (x=0.7, y=0.8) wasformed into a film having a thickness of 1.4 μm. Thereafter, thetransparent thin film of SiO_(x) N_(y) (x=0.7, y=0.8) was formed intoprojections with a pitch (cycle) of 5 μm by a photolithographic methodand a dry etching method thereby to form lattice-like projections andrecesses having rectangular cross-sectional shapes in a planeperpendicular to the longitudinal direction. Thereafter, a polyimidefilm was coated on both substrates, and rubbing treatment was carriedout in the stripe direction of the projections and recesses. Thesubsequent steps were carried out in the same manner as in Example 5 toobtain an optically anisotropic diffraction grating.

Then, in the same manner as in FIG. 5, a phase difference film wasbonded on the second glass substrate. Further, a UV curable acrylicresin was coated thereon, and further a third glass substrate wasmounted thereon, followed by irradiation with ultraviolet rays tolaminate and bond the third glass substrate. Further, with respect tothe entire element, an anti-reflection film was formed on the lightincoming surface and the light outgoing surface to obtain a diffractingelement.

To P-wave (light in a polarization direction parallel to the papersurface in FIG. 5) having a wavelength of 650 nm from a semiconductorlaser, the light transmittance was 95.5% in the going direction.Further, the diffraction efficiency in the returning direction was74.6%.

Example 9

A phase difference film was bonded on the first glass substrate of theoptically anisotropic diffraction grating in Example 8. Further, a UVcurable acrylic resin was coated thereon, and further a third glasssubstrate was mounted thereon, followed by irradiation with ultravioletrays to laminate and bond the third glass substrate. Further, withrespect to the entire element, an anti-reflection film was formed on thelight incoming surface and the light outgoing surface to obtain adiffracting element. In this manner, a construction having the secondsubstrate disposed on a light source side, was obtained.

To P-wave (light in a polarization direction parallel to the papersurface in FIG. 5) having a wavelength of 650 nm from a semiconductorlaser, the light transmittance was 95.6% in the going direction.Further, the diffraction efficiency in the returning direction was78.5%, which was a higher diffraction efficiency than in Example 8.

Example 10

On one surface of the same glass substrate as in Example 5, a SiO_(x)N_(y) film having a thickness of 1.2 μm was formed solely by a plasmaCVD method. At that time, SiH₄, N₂ O and NH₃ were used in a flow rateratio of 1:7.5:2.5 as the atmosphere gas in the plasma CVD apparatus,and the film was formed at the transparent substrate temperature of 300°C. In the SiO_(x) N_(y) film, x and y were about 1.8 and 0.17,respectively.

The SiO_(x) N_(y) film was subjected to etching in a lattice pattern bya photolithographic method and a dry etching method to form projectionsand recesses for a diffraction grating having rectangular cross-sectionswith a depth of about 1.2 μm and a pitch of 9 μm, on the transparentsubstrate.

In the same manner as in Example 5, the two glass substrates were sealedwith an epoxy resin (not shown), and the same liquid crystal wasvacuum-injected. Further, in the same manner as in Example 5, a phasedifference film was laminated and bonded on the second glass substrateby means of a transparent adhesive, and further, a third glass substratewas laminated and bonded thereon by a UV curable acrylic resin. Further,an anti-reflection film was formed to obtain a diffracting element.

Using a semiconductor laser as a light source, P-wave (a polarized lightcomponent parallel to the paper surface in FIG. 4) having a wavelengthof 678 nm was permitted to enter, whereby the transmittance of P-wavewas about 97%. Further, the reflected light (a circularly polarizedlight) from an optical disc (to be provided above the diffractingelement in FIG. 4, not shown) is changed to S-wave (a polarized lightcomponent perpendicular to the paper surface) by a phase differencefilm, and this S-wave was diffracted by the optically anisotropicdiffraction grating, whereby the diffraction efficiency of + first-order(diffraction) light was 34%, and the diffraction efficiency of -first-order (diffraction) light was 34%. In FIG. 4, light from the lightsource is described as S-wave, but in this Example, this becomes P-wave.

As a result, the going efficiency was about 97%, and the go and returnefficiency was about 66% (± first-order (diffraction) light detection).

Example 11

The same structure as in Example 10 was employed except that nopolyimide film was formed. In this case, liquid crystal was alignedsolely by the aligning force of the projections and recesses. In thisExample, the transmittance of P-wave was about 94%, and the diffractionefficiency of + first-order (diffraction) light of S-wave was about 29%and the diffraction efficiency of - first-order (diffraction) light wasabout 28%. As a result, the going efficiency was about 94%, and the goand return efficiency was about 54% (± first-order (diffraction) lightdetection).

Example 12

The operation was carried out in the same manner as in Example 10 exceptthat the SiO_(x) N_(y) film was formed by a reactive direct currentsputtering method. The atmosphere gas in the reactive direct currentsputtering apparatus was N₂ :O₂ =19:1 (flow rate ratio), the temperatureof the transparent substrate was room temperature, and the directcurrent power was 0.2 W. A SiO_(x) N_(y) film wherein x=1.7 and y=0.18,was formed.

In this Example, the transmittance of P-wave was about 93%, and thediffraction efficiency of + first-order (diffraction) light of S-wavewas about 28% and the diffraction efficiency of - first-order(diffraction) light was about 27%. As a result, the going efficiency wasabout 93%, and the go and return efficiency was about 51% (± first-order(diffraction) light detection).

Industrial Applicability

The present invention can be produced with good mass productivity with asubstrate area larger than a berefringence crystal, without using anexpensive material such as berefringence crystal, and a highlight-utilization efficiency can be obtained as compared with anisotropic grating such as a relief-type diffraction grating.

The basic structure such as a pitch, of the optically anisotropicdiffraction grating is defined by the transparent substrate and not byliquid crystal itself, whereby it is excellent in the temperaturecharacteristic and the environmental durability. Further, liquid crystalcan be aligned without providing a film for aligning liquid crystal,such as a polyimide film. With respect to the grating pattern of theoptically anisotropic diffraction grating, a complex grating pattern canbe readily formed with good mass productivity by a CGH (ComputerGenerated Hologram) method using e.g. a mask for exposure.

Further, a phase difference sheet such as a quarter-wave sheet or aphase difference film is laminated within the diffracting element,whereby the productivity is excellent, and the entire element can bemade compact.

According to the present invention, an optical head device can easily beobtained which has a high light-utilization efficiency such that to eachof the incoming light (P-wave) having a polarization in a directionperpendicular to the stripe direction of the optically anisotropicdiffraction grating and the incoming light (S-wave) having apolarization parallel thereto, the light is transmitted at a high lighttransmittance in the going direction, and the light undergoes a desireddiffraction in the returning direction. Further, the opticallyanisotropic diffraction grating to be used for this, can be industriallyproduced with good productivity.

Especially when SiO_(x) N_(y) is used as the transparent thin film, therefractive indices of the projections and recesses and the liquidcrystal can easily be adjusted, and formation of precise shapes can becarried out by a dry etching method.

We claim:
 1. An optical head device, whereby writing of informationand/or reading out of information is carried out by irradiating anoptical recording medium with light from a light source, through adiffracting element, wherein the diffracting element is provided with anoptically anisotropic diffraction grating having lattice-likeprojections and recesses formed on the surface of a transparentsubstrate, and said projections and recesses are filled with a liquidcrystal having an optical anisotropy, and at least the projections amongthe projections and recesses are formed by a transparent thin filmprovided on the surface of the transparent substrate, and saidtransparent thin film is made of SiO_(x) N_(y) (0≦x<2, 0≦y<1.3).
 2. Aprocess for producing an optical head device whereby writing ofinformation and/or reading out of information is carried out byirradiating light from a light source on an optical recording mediumthrough a diffracting element, which comprises coating a transparentthin film on the surface of a transparent substrate for the diffractingelement, then forming lattice-like projections and recesses on thetransparent thin film by a photolithographic method, and filling liquidcrystal having an optical anisotropy in said projections and recesses toform an optically anisotropic diffraction grating.
 3. The process forproducing an optical head device according to claim 2, wherein theoptically anisotropic diffraction grating is formed by filling theliquid crystal between the substrate having the projections and recessesformed thereon and a second transparent substrate.
 4. The process forproducing an optical head device according to claim 2, wherein thetransparent thin film has a refractive index substantially equal to theordinary refractive index or extraordinary refractive index of theliquid crystal.
 5. The process for producing an optical head deviceaccording to claim 4, wherein the transparent thin film is SiO_(x) N_(y)(0≦x<2, 0≦y<1.3).
 6. The process for producing an optical head deviceaccording to claim 4, wherein the transparent thin film is SiO_(x) N_(y)(0.6≦x<1, 0<y<0.9).
 7. The process for producing an optical head deviceaccording to claim 4, wherein the transparent thin film is subjected todry etching to form the projections and recesses.